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Homework Help: Centripetal force?

  1. May 4, 2007 #1
    Centripetal force??

    Does centripetal force actually act on a body?
    I have a question regarding the same here.
    If the satellite is attrated to the earth by the gravitational force F and simultaneously a centripetal force F also acts on the Body. Then the net force acting on the body is:

    My thought over this question:
    Assumption: Zero friction offered by air.
    We can mentally understand that the satellite continues to rotate about the earth without falling towards earth. This means that the net force acting on the satellite must be zero.
    But the answer is wrong?

    This goes to say that in the free body diagram of the satellite there is no centripetal force(centrifugal with opposite direction) to be accounted for.Or is it that we accept the fact that centrifugal is a pseudo force and therefore is not to be accounted for? I am thoroughly confused.
  2. jcsd
  3. May 4, 2007 #2


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    Since the satellite is in orbit in a circular path with a certain cinstant tangential speed, there must be a centripetal acceleration (v^2/r) due to its constanrly changing direction along the tangent of its path. You can't have an acceleration without a force, so there must be a force acting on it, in the direction of the centripetal acceleration. What is that force??
  4. May 5, 2007 #3
    Action and reaction force doesn't act on the same object. When you push an object, you exert a force - so that it began to move. According to newtons third law action and reaction forces exist in pair, so there must be a reaction force. The reaction force acts on you. If both action and reaction force acted on the object you have pushed, it shouldn't have moved- since they are both equal and opposite.

    Centipetal force is a force that is required to keep an object moving in circular motion. It can result from anything- from graviational to frictional force. Here centipetal force is provided by the graviational attraction between the satellite and the earth........
    Last edited: May 5, 2007
  5. May 5, 2007 #4
    So therefore the centripetal force(with respect to an inertial frame) is basically the net unbalanced force acting on a body along the radius towards the centre.
  6. May 5, 2007 #5
    You are right. ;-)
  7. May 5, 2007 #6


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    A satellite in orbit, or any mass orbiting another mass, is constantly accelerating. Even if the tangential speed did not change, the orbiting mass is constantly changing direction.
  8. May 5, 2007 #7

    Chi Meson

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    Mostly good explanations all along, I just wanted to make it clear that:
    the gravitational force IS the centripetal force in this situation (assuming a perfectly circular orbit). The centripetal force is not a type of force itself; rather, it is a designation of some other recognizable force (such as gravity, or perhaps tension if it's a ball on a string). The word centripetal designates that this other force is point toward the center of a circular path (and is therefore pointing pependicularly to the motion at that moment, and is therefore doing no work).

    I dislike the question in the OP because it erroneously suggests that there are two forces on the satellit. It does this by pushing in the word "simultaneously." There is only one force on this satellite, it is the gravitational force, which is acting "centripetally."
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